This invention pertains generally to the field of microelectromechanical systems, microplasmas and particularly glow discharges, and to spectroscopy using glow discharges.
Because of the risk of industrial and biochemical pollution of potable water sources, diagnostic tools and systems that can provide rapid on-site tests for contaminants are of increasing importance. At present, water quality assessment is a relatively elaborate process, requiring sample transportation to centralized locations at which laboratory analysis is conducted. This analysis is typically done with a plasma spectrometer, a machine which varies in size from tabletop models to room size systems. In a plasma spectrometer, the water sample is sprayed into a high frequency radio frequency (RF) inductively coupled plasma, and atomic transitions of the impurities are analyzed to determine the composition and quantity of the water impurities. This process is similar to the detection of gases in plasmas.
In addition to plasma spectrometry, various other techniques are available for the diagnosis of gas and water impurities, several of which have been miniaturized using microelectromechanical systems (MEMS) technology. One example is mass spectrometry which measures the ratio of ion mass to charge for gases using various techniques. Quadrupole mass spectrometers have an ion source, a quadrupole electrostatic lens to focus the ion flow, and an array of detectors. Ions with smaller mass to charge ratios are electrostatically deflected more than larger mass ions. A micromachined quadrupole mass spectrometer has been constructed with 500 micron diameter quadrupole electrodes. Time of flight mass spectrometers ionize gas atoms quickly, accelerate the ions, typically electrostatically, and measure their time of flight, which is a function of ion mass. Ion mobility mass spectrometers have the capability of operating at atmospheric pressure. These devices ionize gas using DC or RF voltage, or lasers. Separation of species is based on their different mobilities in a background gas. Such RF-based ion mobility spectrometers have also been miniaturized. Gas chromatographs separate different gases in a carrier gas flowing through a heated tube by exploiting differences in mobility for analysis. Detection mechanisms for chromatography vary, with gas ionization and spectral analysis being common. Efforts have also been made to miniaturize such gas chromatographs.
Microplasmas have been the subject of increased research in recent years. For example, DC microplasmas for silicon etching have been ignited between thin-film metal electrode features patterned on the substrate to be etched, C. G. Wilson, Y. B. Gianchandani, xe2x80x9cSilicon Micromachining Using In-Situ DC Microplasmas,xe2x80x9d JMEMS, Vol. 10, No. 1, March, 2001, pp. 50-54. Efforts have been directed at miniaturizing inductively coupled plasmas for gas spectroscopy, J. A. Hopwood, xe2x80x9cA Microfabricated Inductively Coupled Plasma Generator,xe2x80x9d JMEMS, Vol. 9, No. 3, September, 2000, pp. 309-313, and to utilize DC microplasmas as an optical emission source for gas chromatography, J. C. T. Eijkel, H. Stoeri, A. Manz, xe2x80x9cA DC Microplasma on a Chip Employed as an Optical Emission Detector for Gas Chromatography,xe2x80x9d Anal. Chem., Vol. 72, June, 2000, pp. 2547-2552. Atomic transitions of metallic impurities are typically best detected from spectroscopic analysis of DC plasma emissions. See, N. W. Routh, xe2x80x9cDCP Advantages,xe2x80x9d Applied Research Laboratories Application Reports. Efforts have also been made to employ a water sample as a cathode, with a metallic anode for spectroscopic use. T. Cserfalvi, P. Mezei, xe2x80x9cEmission Studies on a Glow-Discharge in Atmospheric Air Using Water as a Cathode,xe2x80x9d J. Phys. D (Appl. Phys.), Vol. 26, 1993, pp. 2184-2188; R. K. Marcus, W. C. Davis, xe2x80x9cAn Atmospheric Pressure Glow Discharge Optical Emission Source for the Direct Sampling of Liquid Media,xe2x80x9d Anal. Chem., Vol. 73, 2000, pp. 2903-2910. Such systems have also been implemented in a MEMS device. G. Jenkins, A. Manz, xe2x80x9cOptical Emission Detection of Liquid Analytes Using a Micro-Machined DC Glow Discharge Device at Atmospheric Pressure,xe2x80x9d xcexcTAS, October, 2001, pp. 349-350.
The glow discharge apparatus in accordance with the invention utilizes liquid electrodes which allow spectrometric analysis of liquid samples and particularly water samples for determining contaminants in the water. The invention may also be utilized as a micro light source that provides light output at visible or non-visible wavelengths that can be selected by selection of the liquid utilized in the electrodes or of the materials dissolved or suspended in the electrode liquids.
The glow discharge apparatus in accordance with the invention includes a substrate with a top surface and a cathode electrode and an anode electrode formed thereon. At least the cathode electrode includes a cathode terminal port formed as a depression in the substrate which is formed to hold a liquid sample to be analyzed or otherwise sputtered or evaporated into the glow discharge. The anode electrode also preferably includes an anode terminal port spaced from the cathode terminal port by an inter-electrode gap, with the substrate being electrically insulating between the anode electrode and the cathode electrode. An anode electrical conductor on the substrate is electrically connected to the anode electrode and a cathode electrical conductor on the substrate is electrically connected to the cathode electrode to allow a voltage to be applied from a voltage source between the anode electrode and cathode electrode. The applied voltage creates a glow discharge in the gap over the inter-electrode gap which results in sputtering and/or of the liquid in the cathode terminal port into the gap and its excitation by the discharge in the gap. The spectrum of the light emitted from the discharge will depend on the constituents of the liquid sputtered or evaporated into the gap, allowing spectroscopic analysis of the material of the liquid in the cathode terminal port. The spectrum of the light emitted will also depend on the ambient pressure (which may also be above or below atmospheric pressure if the apparatus is enclosed) and on the power density of the discharge. Spectroscopic analysis can be carried out, for example, by a spectrometer which is coupled to the light in the discharge by an optical fiber or by a microspectrometer formed with the glow discharge apparatus. Alternatively, by choosing the constituents of the liquid in the cathode electrode, the light emitted in the discharge at the gap can be selected to provide a convenient micro light source of selectable wavelengths.
The cathode electrode preferably includes a reservoir formed in the substrate and a channel extending from the reservoir to the cathode terminal port, with an insulating layer covering the channel so that only the terminal port is in electrical communication with the glow discharge in the inter-electrode gap. The anode electrode may be formed in a similar fashion, having a reservoir and a channel extending to the anode terminal port with an insulating covering over the channel. The anode and cathode conductors may be formed on the top surface of the substrate extending from the anode and cathode channels to positions spaced away from the anode and cathode electrodes. Insulating material may be formed on the top surface of the substrate covering the conductors with openings formed therein at which electrical leads may be connected to the anode and cathode conductors. The anode electrode may also be formed as a solid conductor having a terminal portion exposed on the surface of the substrate spaced from the cathode terminal port.
A particular advantage of the invention in spectroscopic analysis of water samples is that the water sample and its impurities are effectively sputtered into the glow discharge during operation, eliminating the need for spraying which has otherwise been necessary for detecting non-volatile contaminants. Moreover, for use in either spectroscopic analysis or as a light source, the invention has the advantage that electrode wear is eliminated since the active portions of both electrodes may be formed of liquids and thus any electrode material evaporated or burned away is effectively replenished by liquid flow from the reservoirs. Furthermore, a continuous glow discharge can be sustained in air at atmospheric pressure, eliminating the need for a vacuum system. The utilization of two liquid electrodes has the additional advantage that spurious contamination by the anode material is avoided.
The apparatus can be operated in a calibration mode, e.g., with the anode filled with pure water (modified to an appropriate pH level) while the cathode can be filled with the water being tested. Operating the device with DC voltage applied with correct polarity (negative voltage on the cathode) which gives spectroscopic readings on impurities in the sample. Operating with reverse polarity will provide a baseline reading of the pure water (or water with known impurities) in the anode. In addition, the device can be operated with a square wave in a xe2x80x9cchoppingxe2x80x9d mode, to provide continuous impurity and baseline readings.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.